linux-kernel - Re: [PATCH 07/19] sched/fair: Track LLC-preferred tasks per runqueue

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Message-ID: <ab01cb87-71f6-4893-833e-136e7acd777d@intel.com>
Date: Tue, 28 Oct 2025 23:15:53 +0800
From: "Chen, Yu C" <yu.c.chen@...el.com>
To: K Prateek Nayak <kprateek.nayak@....com>
CC: Vincent Guittot <vincent.guittot@...aro.org>, Juri Lelli
	<juri.lelli@...hat.com>, Dietmar Eggemann <dietmar.eggemann@....com>, "Steven
 Rostedt" <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, Mel Gorman
	<mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>, "Madadi Vineeth
 Reddy" <vineethr@...ux.ibm.com>, Hillf Danton <hdanton@...a.com>, "Shrikanth
 Hegde" <sshegde@...ux.ibm.com>, Jianyong Wu <jianyong.wu@...look.com>,
	"Yangyu Chen" <cyy@...self.name>, Tingyin Duan <tingyin.duan@...il.com>, Vern
 Hao <vernhao@...cent.com>, Len Brown <len.brown@...el.com>, Aubrey Li
	<aubrey.li@...el.com>, Zhao Liu <zhao1.liu@...el.com>, Chen Yu
	<yu.chen.surf@...il.com>, Adam Li <adamli@...amperecomputing.com>, Tim Chen
	<tim.c.chen@...el.com>, <linux-kernel@...r.kernel.org>, Tim Chen
	<tim.c.chen@...ux.intel.com>, Peter Zijlstra <peterz@...radead.org>, "Ingo
 Molnar" <mingo@...hat.com>, "Gautham R . Shenoy" <gautham.shenoy@....com>
Subject: Re: [PATCH 07/19] sched/fair: Track LLC-preferred tasks per runqueue

On 10/27/2025 2:04 PM, K Prateek Nayak wrote:
> Hello Tim,
> 
> On 10/11/2025 11:54 PM, Tim Chen wrote:
>> @@ -3999,6 +4038,7 @@ account_entity_enqueue(struct cfs_rq *cfs_rq, struct sched_entity *se)
>>   		struct rq *rq = rq_of(cfs_rq);
>>   
>>   		account_numa_enqueue(rq, task_of(se));
>> +		account_llc_enqueue(rq, task_of(se));
>>   		list_add(&se->group_node, &rq->cfs_tasks);
>>   	}
>>   	cfs_rq->nr_queued++;
>> @@ -4010,9 +4050,14 @@ account_entity_dequeue(struct cfs_rq *cfs_rq, struct sched_entity *se)
>>   	update_load_sub(&cfs_rq->load, se->load.weight);
>>   	if (entity_is_task(se)) {
>>   		account_numa_dequeue(rq_of(cfs_rq), task_of(se));
>> +		account_llc_dequeue(rq_of(cfs_rq), task_of(se));
>>   		list_del_init(&se->group_node);
>>   	}
>>   	cfs_rq->nr_queued--;
>> +
>> +	/* safeguard to clear the cache aware data */
>> +	if (!parent_entity(se) && !cfs_rq->nr_queued)
>> +		reset_llc_stats(rq_of(cfs_rq));
> 
> Instead of relying on reset_llc_stats() hack, I think a better approach
> would be to have a "p->se.llc_sched_active" flag similar to how uclamp
> has "uc_se->active" and we set this in account_llc_enqueue() which will
> still check for sched_cache_enabled() but account_llc_dequeue() would
> only check for "p->se.llc_sched_active" to decrement the stats and then
> unset the flag.
> 
> That way, we cannot have an imbalanced accounting. Thoughts?
> 

I suppose what you mean is to avoid the race condition between
enabling sched_cache and EQ/DE_LLC, similar to uclamp:

         enqueue(taskA)
         // sched_cache gets enabled
         enqueue(taskB)
         dequeue(taskA)
         // Must not decrement rq->llc_pref for taskA
         dequeue(taskB)

We'll think more about this.

thanks,
Chenyu